1. Field of the Invention
The present invention generally relates to electron beam (e-beam) lithography apparatus and, more particularly, to performance of proximity correction of exposure patterns produced thereby to compensate exposure of a portion of a desired pattern which is in the vicinity of another portion of the desired pattern.
2. Description of the Prior Art
In the past, many types of electronic devices have utilized structures including patterns of conductive material on a substrate to form connections between electronic devices such as resistors, capacitors and transistors either mounted or formed thereon. Printed circuit boards are an example of such structures. More recently, such structures have been used in integrated circuits, multi-layer ceramic (MLC) devices and similar devices in which complex circuits are formed at very high density. The density and size at which the connection patterns are formed is particularly critical in integrated circuits.
Such conductive patterns are typically formed, for efficiency and general reliability regardless of the density or size of features of the conductive pattern, by applying a pattern of resist to a layer of conductive material and then etching away areas of the conductive material where resist is not present. Particularly at small sizes, patterning of the resist is typically done by exposing a layer of a resist to a pattern of radiation such as light and then removing either the exposed or unexposed portions of the resist. As feature size has decreased, different types of radiation have been used to obtain higher exposure accuracy and resolution. Exposure with charged particle beams such as electron beams has become widely used for high density integrated circuits.
In electron beam (e-beam) exposure systems, often referred to as "e-beam tools" or simply "tools", the exposure is made by calculating the locations and sizes of portions of the desired exposure pattern. Several approaches have been made to decomposing the desired pattern into shapes which are generally rectangular. U.S. Pat. Nos. 4,878,177, to Ikenaga, and 4,482,810 to Cooke are exemplary of such arrangements. For the purpose of conveying an understanding of this invention, the decomposition of shapes will be considered as decomposition into rectangular pattern portions, tiling these portions with smaller rectangles (e.g. interior and exterior rectangles) and then tiling the smaller rectangles with rectangular exposure spots which are written sequentially at extremely high speed. These rectangular exposure spots or spot rectangles have a maximum dimension, called a maxspot size or simply maxspot in both coordinate directions (hereinafter referred to as horizontal and vertical for convenience) and, where edge location is critical, are usually limited in dimension in one of the coordinate directions to the dimension at which best focus of the tool is obtained. The other dimension of the exposure spot will usually be the full maxspot size unless a smaller spot is required. Such a smaller spot is referred to as a remainder spot. There is also a practical limitation on the minimum size of a remainder spot, referred to as a "sliver", and a constraint is placed on rectangle division that prevents division being made which would produce such a dimension. This constraint is necessary since it is possible to compute a valid exposure dose for a region which is smaller than the e-beam tool can accurately produce.
Incidentally, the technique of proximity correction referred to herein as the prior technique, method, process and the like are specifically not admitted to be prior art as to the present invention. In order to convey a better understanding of the nature and meritorious effects of the invention, the prior technique is so identified as a matter of convenience to permit a better understanding of the invention by allowing the invention to be contrasted therewith.
In the prior technique, the locations and sizes of each of the individually exposed spots are typically determined by a computer and the tool accordingly controlled thereby to develop the high speeds required in view of the extremely large numbers of spots to be written and the desired throughput of the tool. Data processing requirements are reduced substantially by dividing the desired exposure pattern into rectangular portions which may each contain one or more exposure spots. Tiling of such rectangles with exposure spots in a sequential order is a very orderly procedure which can be done autonomously under computer control once the bounds of each rectangular portion are established.
As with any radiation sensitive material, exposure time is cumulative. Although exposures may be readily determined and controlled in an ideal manner, an inherent problem in e-beam exposure tools is secondary emission and forward scattering (e.g. localized dispersion of the beam, measurable at the tool, as it travels to the target due to mutual repulsion of electrons in the beam, aberrations of electron lenses in the tool, variations in electron velocity and quantum effects) as well as some degree of backscattering of electrons from the resist target (or underlying structure) being exposed. At a nominal acceleration voltage of about 50 KV needed to provide an adequate e-beam current for rapid exposure of the ideal pattern, a significant further exposure occurs in the vicinity of each exposed spot due to the secondary emission and backscattering of electrons. This effect will occur over a generally circular area having a radius which will be referred to hereinafter as the scattering distance. In fact, however, the distribution of impinging electrons resulting from scattering and/or secondary emission may be statistically described and extend, to some degree, beyond this radius. The radius referred to hereinafter is the distance beyond which cumulative exposure effects can be neglected. It is also to be understood that both effects of backscattering and secondary emission are to be considered together and references hereinafter to secondary emission or proximity effects should be considered as including both secondary emission and electron scattering effects, such as forward scattering, as discussed above, which also contribute to the additional exposure.
Incidentally, the exposure profile along the radius of the area over which secondary emission effects occur is usually highly non-linear unless adjusted by e-beam acceleration voltage as in Ban et al, U.S. Pat. No. 4,500,789, and the exposure due to scattering effects only slightly changes the shape of the exposure distribution curve. The shape of this curve may be considered to a greater or lesser degree in the proximity correction algorithm utilized to determine the actual exposure values. However, the actual proximity correction algorithm employed is not important to the practice of the present invention.
When it is considered that exposure levels must usually saturate the sensitized material to obtain full contrast, it can be understood that overexposure by secondary emission and scattering can easily cause "blooming" of the pattern. While blooming may not seem to be important at the interiors of relatively large shapes, it causes loss of geometrical definition and resolution of the pattern at edges thereof and is therefore particularly objectionable when fine pattern detail is to be produced. Therefore, there is often a need to compensate exposure spots at both the interior and the edges of patterns; the latter often being referred to as exterior rectangles, determined by a process called sleeving, for the secondary emission exposure from exposure of interior spots.
An exemplary sleeving technique is shown in U.S. Pat. No. 4,943,729, to Ando et al. IBM Technical Disclosure Bulletin, Volume 20, No. 9, pp. 3809-3812, Partitioning E-Beam Data For Proximity Corrections, by Chang et al. also teaches a technique similar to sleeving for partitioning pattern data by forming a large perimeter around a shape and modifying exposure dose where the enlarged perimeters meet for partial proximity correction.
U.S. Pat. Nos. 4,426,584 and 4,504,558 to Bohlen et al. teach proximity corrections in a projection e-beam lithography arrangement by using multiple masks to alter exposure dose. Other approaches to proximity correction are taught by U.S. Pat. Nos. 4,895,760, to Nissan-Cohen, which applies windage to a pattern in accordance with multiple exposures, and 4,812,962, to Witt, which involves stepping across a pattern to identify neighboring areas of a given shape, 4,816,692, to Rudert, Jr., which is directed to a pattern splicing system, and 4,099,062, to Kitcher, which is directed to multiple overlapping exposures at reduced exposure levels.
While these techniques have yielded substantial improvement in pattern accuracy, such techniques often require time-consuming multiple exposures and have not accounted for the fact that, at the present state of the art, the optimum focus dimension and, hence, the potential feature size of the pattern has been reduced below the distance over which secondary emission and scattering causes cumulative exposure. Therefore, mere sleeving compensation is inadequate to fully exploit the currently available resolution of e-beam tools. Further, at such a scale, several factors cause any attempt at a further level of compensation to become computationally formidable. This can be readily understood from the observation that, potentially, each spot exposure affects and is affected by every other spot exposure within a radius which corresponds to the secondary emission scattering exposure distance. Further, as exposures are progressively made over the writing area, the exposure of a spot may cause alteration of exposure of a second spot within the scattering exposure distance and, thus, in turn, affects the exposure of a spot beyond the scattering exposure distance. The same is true for each spot rectangle in every rectangular shape in the desired pattern. In either case, the computational overhead increases enormously as the number of related exposure areas (either spots or rectangles) is increased, as is the case when spot size is reduced.
Exemplary known computational techniques are taught by Eichelberger et al., U.S. Pat. No. 4,687,988, IBM Technical Disclosure Bulletin Vol. 22 No. 11, pp. 5187-5189, Data Zoning in the Proximity-Effect Correction Technique, by M. Parikh, and Representative Figure Method for Proximity Effect Correction, by Abe et al., Japanese Journal of Applied Physics, Vol. 30, No. 3B, March, 1991, pp. L528-L531. All of these computational techniques require either such a large quantity of data that computation cannot be efficiently done or involve simplifying assumptions which reduce the accuracy of the computational result below an acceptable level.
It is to be understood that the ability to make compensating exposure corrections is therefore limited by the ability to organize and process such potentially massive amounts of data. It is for this reason that the resolution of known processes has remained poor. That is, the amount of data which can be processed in a practical fashion is limited. Therefore, the number of rectangles on which correction can be made is relatively low and the average size of correctable rectangles remains large, resulting in poor resolution of the exposure compensation process. This limitation of prior exposure compensation processes exists regardless of the proximity correction algorithm employed.